Process of making a ceramic element usable in surface-gap igniters



' square inch (gage) pressure. point of ignition also may vary widely, for example, from about -65 F. to about 2000 F. There is commonly PRGCESS F MAKENG A CERAMIC ELEMENT USABLE IN SURFACE-GAP IGNITERS Shay .I. Sheheen, Ithaca, N.Y., nssignor to General Laboratory Associates, Inc, Norwich, N.Y., a corporation of New York No Drawing. Application October 3, 1956 Serial No. 613,598

2 Claims. (Cl. 204-29) The present invention relates to a process of making a ceramic element usable in surface-gap igniters and product thereof. Such devices as herein disclosed are particularly useful in igniting fuel and air mixtures in internal combustion engines and particularly in jet engines of the type used in aircraft. Various phases of the invention have, however, much wider utility, for example, in spark-producing devices in general.

Spark plugs or igniters of the common type used on internal combustion engines have a pair of electrodes separated by an air gap in which the spark is produced. Such igniters commonly include a member of insulating material which is disposed between the electrodes and has a sunface adjacent to the air gap. When in use, such igniters are at times subject to a condition commonly termed fouling, characterized by the formation of a deposit, usually carbonaceous in character, on the insulator surface. The carbon deposit provides a more or less electrically conductive path in parallel with the spark gap and weakens the spark, or may, if sufiiciently conductive, prevent the formation of a spark.

It has also been proposed to provide an igniter of the so-called surface gap type. In such igniters, the electrodes are separated by a semi-conductive member, rather than a member of insulating material. Portions of the surface of this member, including particularly the metal-to-semi-conductor contact area, are believed to be heated by the electric current when a discharge is initiated between the electrodes, and are thereupon believed to give off electrons which aid in breaking down the gap, so that the main discharge follows as a spark between the electrodes and just above the conductive surface. Surface gap igniters have the advantage of a lower voltage requirement for a given spark energy. Furthermore, the heating of the conductive surface during the discharge tends to burn off any fouling deposit. However, in igniters of this type constructed in accordance with the teachings of the prior art, the energy dissipated in the atent spark may vary considerably under fouling conditions,

as the energy dissipated in the fouling deposit varies with its thickness and composition.

In internal combustion engines ignition takes place following a compression stroke in a chamber having a high temperature, a high pressure, and a reasonably uniform mixture of fuel and air. Under such conditions, successful ignition of the mixtures is comparatively easy. As long as some kind of spark occurs, the fuel mixture will ignite.

In jet engines, however, the conditions are quite different and the problem of producing satisfactory ignition of the fuel is considerably more complicated. The pressure may vary widely, for example, between about 3 inches of mercury (absolute) and 250 pounds per The temperatures at the a great amount of turbulence in the neighborhood of the igniter, and there is a wide variation in the fuel-air ratio of the mixture presented for ignition.

Because of these considerations, it is desirable in such engines to provide ignition apparatus which will produce sparks of uniformly high energy.

In addition to the low temperature requirements during starting conditions, an igniter for a jet engine must be able to withstand very high temperatures, for example, of the order of 1800-2000 F., during operation of the engine.

The term semi-conductive materials as used in this application is a well recognized generic term for a large group of materials which are neither good electrical conductors nor good electrical insulators. These materials commonly have a negative temperature coefiicient of electrical resistance. In other words, their resistance decreases as their temperature increases. This characteristic is just the opposite of the corresponding characteristic of electrical conductors, the resistance of which commonly increases as their temperature increases.

In a surface gap type of igniter, the energy dissipated in the spark, as contrasted to the energy dissipated in the conducting surface, is believed to be determined to a great extent by the emission of electrons from the surface. The greater the quantity of electrons emitted, the quicker the gap through the air breaks down, and the greater the quantity of energy dissipated in the gap rather than in the conducting surface.

The present invention is particularly intended to provide a surface gap type igniter wherein the surface will be semi-conductive or partially conductive and particularly wherein the conductivity of this surface portion will be increased sufficiently with respect to that of prior art devices of this general nature as to permit the initiation of a spark or discharge at a lower breakdown voltage than has been possible with prior art devices.

The ceramic element made according to the teachings of a copending application which is owned in common with the present application may be used as a starting material or starting point for the process of the present invention. This copending application is Serial No. 603,971, filed August 14, 1956, and entitled impregnated Semi-Conductive Ceramic Body, Process of Making it and I gniter Including Such Body, now Patent No. 2,- 861,014, issued November 18, 1958. Alternately, the partially formed ceramic element prepared in accordance with the first portion only of the teachings of said copending application may be used as the starting point or starting material for the present process. In any event, the present process has as its general purpose to provide a way to decrease the resistance and more or less correspondingly to lower the breakdown voltage for starting a discharge across a given area or space along a treated surface of a ceramic body. In this connection, the ceramic body of the present invention may be incorporated in an igniter device as disclosed, for example, in the copending application aforesaid and may be energized, for example, by a system as disclosed in the US. Patent to John V. McNulty, No. 2,716,720, issued August 30, 1955.

Summarizing the present invention, it comprises a process of preparing a ceramic element usable in surfacegap igniters and the like starting with a ceramic body which has at least a part of its surface which is semiconductive as that term is defined hereinabo-ve and which is rendered more conductive by the process of the present invention. Furthermore, the starting body according to the present invention must contain copper in an oxide form as hereinafter defined. In accordance with this process some, usually at least about half but substantially less than all, of this copper oxide is reduced to metallic copper by electrochemical reduction, following which the conductivity of this partially conductive surface portion is further augmented by electrochemically depositing copper onto a partially conductive surface and/or onto the reduced copper on and accessibleto this surface, by having the semiconductive surface connected as a cathode in an electrochemical cell,-the electrolyte of which contains copper ions.

Following this (and the process may be terminated at this point if desired), it is sometimes found that the resistance of the partially conductive surface has been reduced too far. If this is so, the present invention contemplates the controlled increase of this resistance by partially oxidizing some of the reduced copper at and adjacent to the conductive surface by exposing the body to an oxidizing environment at a relatively high temperature and controlling the amount of oxidation by controlling the temperature and the duration of the heating and/ or exposure to the oxidizing environment.

It is further found in some instances, where the ceramic element of the present invention is to be used at high temperatures andin an oxidizing environment, that it may be desired substantially to stabilize the resistance at the value to which it was adjusted either by reduction of copper oxide or such reduction followed by a partial oxidation. In these circumstances, it may be desired to plate onto the conductive surface or a part thereof some metal which is less subject to oxidation to high temperatures than is copper. 'The metals actually tried for this purpose are nickeland'rhodiurn. These several expedients will be explained more in detail hereinafter.

Other and more detailed objects and features of the present invention will become apparent from the following particular description of certain preferred embodiments thereof.

Turning now to the details of the present invention, the first factor to be considered is the requirements for the starting material or ceramic body. This starting material or ceramic body is one usually having substantially the shape and size desired for the ceramic element to be made and also having at least one semi-conductive surface portion which is formed at least in part of a copper oxide. This descriptive phrase is intended to include not only the final product made by the process described and claimed in the copending application of Sheheen and Mooney, Ser. No. 603,971, aforesaid, but also the product, for example, of the first stage of the twovstage process set out in that application, i.e., wherein a ceramic body which may consist essentially of alumina has a selected portion only of its surface treated or impregnated with a material including copper oxide. It is further contemplated that a ceramic body of any desired ceramic material, including one which consists essentially of alumina but which contains no copper oxide, may have suitably secured thereto a layer or portion containing or consisting of an oxide of copper or having a surface portion containing or consisting of an oxide of copper. The only particular requirement therefore is that the starting ceramic body shall have an oxide of copper at and/or accessible to a surface portion thereof, so that it may be reached by an aqueous electrolyte for the purpose of electrolytically reducing at least a part of the copper oxide to metallic copper.

It will be noted that throughout this application the term copper oxide or a copper oxide is used. By either or both of these terms are meant either orboth oxides of copper, i.e., Cu O and CH or any mixture thereof. It is recognized that in reducing CuO, this material probably passes through the lower valence state of 01 0 in the course of its reduction to metallic copper. 1 The first step of the process in accordance with the present invention is the reduction of some of the copper oxide present andaccessible to the semi-conductive surface to the state of metallic copper. The purpose of this III-generally set out above is to decrease the electrical resistance across a part at least of the semi-conductive ably about 3 to about 6 amperes per square foot. In such a bathla satisfactory amount of copper can be .de-

surface and to decrease the breakdown voltage for an igniter in which the ceramic element of the present invention may be used. The original breakdown voltage for a conventional gap distance of about 0.03 to 0.04 inch may be in the order of magnitude of 800 to 1000 volts; whereas after the reduction step, which is the first step of the present process, this breakdown voltage may be reduced to the range of about 500 to about 800 volts.

The first step of the process is effected by conventional electrochemical reduction usually, for example, wherein the conductive surface portion at least of the ceramic body is used as the cathode of an electrochemical cell. The use of any conventional chemical means for effecting reduction of a substantial amount of the copper oxide to metallic copper is to be considered within the purview of the present invention.

If the reduction operation is to be effective in a cell and separate and distinct from other cells hereinafter referred to, the conductive portion of the ceramic body to be reduced may be connected as a cathode in the cell, the electrolyte of which consists essentially of aqueous sulphuric acid, for example, of about 20% H 50 concentration. In such conditions it has been found possible to effect a reduction of at least about one-half but substantially less than all the copper oxide accessible to the surface in contact with the electrolyte in about one to two minutes electrolysis time and with a cathode current density of about 2 amperes per square foot. In any event, it is preferred that at least about one-half but substantially less than all the copper oxide accessible to the surface which is in contact with the electrolyte shall be reduced by the electrolysis to metallic copper.

The next step of the process according to the present invention is further to reduce the resistance of the partially conductive surface portion of the ceramic element, so as further to reduce the breakdown voltage aforesaid. This step is effected by plating onto a part of the semi-conductive surface, and probably onto the reduced copper particles thereon a further amount of copper by more or less conventional electroplating techniques.

In the event that the reduction step of the process is carried on in a separate electrochemical cell which is contemplated as aforesaid, then the copper plating or electro-deposition of copper may be carried on by immersing the body or at least the semi-conductive surface portion thereof into the electrolyte of a copper plating cell in which the semi-conductive surface portion is electrically connected as the cathode of the cell. The electrolyte in this cell may be any one having copper ions in an aqueous solution. The cell arrangement and the characteristics of the electrolyte other than those particularly described herein may be substantially conventional for copper plating.

In the event that a cyanide type plating bath is des1red,'the electrolyte may be an aqueous solution including about 3 ounces per gallon of copper cyanide, about 4 to 4% ounces per gallon of sodium cyanide, about 2 ounces per gallon of sodium carbonate, and such additional water-soluble salts, if any, as may be required to adjust the pH of the solution to a desired value, which is preferably at least about 10.

, It is required, of course, that the bath must be kept on the alkaline side of neutral as there is dissolved cyanide therein, which would result in the evolution of high- 1y poisonous hydrocyanic acid if the solution were acid. Furthermore,-i t has been found that the bestplating etficiencies are obtained with a pH of at least about 10, and preferably in the range of about 10 to about 13. The current density for use under these conditions is from about 2.5 to about 15 amperes per square foot and preferposited in about 2 to 10 minutes, depending on the area exposed. It will be understood that the time and current aaaaaso density may be reciprocally varied and substantially equivalent results obtained, i.e., if the time is to be reduced, the current density should be proportionally increased and vice versa. In such a cell it is usually found convenient to use a copper anode, although other types of anodes are feasible and operative, nickel, chromium, and rhodium having been successfully used. These materials are not to be considered as inclusive of all possible materials which could be used to have the process fully operative, as in general the requirements are merely those of conventional copper plating.

In the event it is desired to use an acid bath rather than acyanide bath, a conventional acid copper plating bath may be used. Such a bath may be made up, for example, by dissolving in water CuSO -5H O, 24 ounces per gallon and added sulphuric acid (H 80 about 6 ounces per gallon. Such a bath may be operated at room temperature with a cathode current density in the order of magnitude 25 to 60 amperes per square foot. The same time range as aforesaid may be used and again time and current density may be reciprocally varied to give equivalent results.

The result of the plating operation is effective to reduce the resistance across the partially conductive surface, so as to have a breakdown voltage of about 200 to about 500 volts when the device is used in an igniter as is disclosed in the copending application aforesaid.

It is further particularly contemplated that the first two process steps of reducing copper oxide and plating copper may be carried out in the same electrolytic cell and in the stated sequence, i.e., the reducing operation to take place first and thereafter copper ions from the electrolyte to be plated onto the partially conductive surface portion of the cathode. It has been found that these process steps do in fact occur in this order, so that the first two steps of the process have been and may be carried out in a single cell if desired.

In some instances it may be that the process aforesaid of reducing the resistance and the breakdown voltage will have been carried on too far, so that the resulting product will have a lower resistance and a lower breakdown voltage than the respective values desired. If this occurs, it is found that the resistance may be suitably increased and the breakdown voltage more or less correspondingly increased by partially oxidizing some of the copper on the partially conductive surface of the ceramic element. This can be accomplished by raising the temperature of the ceramic body preferably to a temperature in the range of about 1000 F. to about 1200 F. and holding the temperature in this range while the ceramic body is exposed to an oxidizing atmosphere, such as atmospheric air, for about 10 minutes. It will be understood that the degree of oxidation can be controlled by controlling the temperature and time, with higher temperatures and longer times both resulting in greater oxidation and correspondingly greater increases in electrical resistance and in breakdown voltage. The use of this partial oxidation step, when necessary or desirable, may be considered a part of the present invention; although it will be understood that this step is optional and may be omitted and the desired reduction in resistance and in breakdown voltage from the values of those characteristics in the starting body may be attained solely by the reduction and plating steps above described.

In view of the fact that it is possible as aforesaid to increase the resistance by partial oxidation of the copper at and accessible to the partially conductive surface, it will be seen that the resistance and breakdown voltage characteristics are not really stable if the ceramic element produced by the process thus far particularly described is exposed in an oxidizing environment under conditions of relatively high temperature. Inasmuch as both conditions may occur simultaneously in the use of the device in igniters for jet engines, it may and often is desirable to follow the steps of the process thus far particularly described by a further process step designed to stabilize the resistance even under these conditions and similarly to stabilize the breakdown voltage.

For this purpose advantage is taken of the fact that certain metals, such as nickel and rhodium, are more nearly proof against oxidation at relatively high temperatures than is copper, even though they are still somewhat oxidizable under these conditions. However, the tendency to oxidize a surface protected by a layer or plating of a metal as nickel or rhodium is far less than if that surface were of metallic copper. For this reason, therefore, it is proposed in accordance with the present invention to stabilize the resistance and breakdown voltage characteristics of the ceramic element produced as aforesaid by plating onto the partially conductive surface a. layer of one of the metals, nickel or rhodium. Either of these metals may be plated onto the metallic copper on the semi-conductive surface from a conventional plating bath for the metals respectively, wherein the metal in question exists as water-soluble electrolyte, providing ions of the respective metals in aqueous solution.

In the case of rhodium, the electrolyte may be made up using a conventional and commercially available solu tion of a water-soluble rhodium salt, which may be plated from an acid solution. Successful results have been attained from using such a bath.

One such electrolyte for nickel may be made up as an aqueous solution containing 32 ounces per gallon of NiSO -7H O, 2 ounces per gallon NiOl -6H O and 4 ounces per gallon H BO An electrolyte of this composition is effective at a pH value of about 6 and may be used either at about room temperature or at a temperature of about -130 F. It is preferable that the nickel plate thus applied shall be about .002 inch in thickness, so as to provide a sufficient thickness to compensate for a possible oxidation of some nickel on the surface, while still retaining a sufficient amount of nickel to protect the subjacent copper from oxidation. Such a coating has been obtained using a conventional nickel plating bath as aforesaid and with a time and current density sulficient to give this plating. The time and current density may be reciprocally adjusted as aforesaid. The actual values for current density are not given here as the areas to be plated are so small that measurement thereof is diflicult and somewhat uncertain at best. It is found, however, that the thickness of nickel plate as aforesaid is sutficient to obtain desired results notwithstanding that some of this nickel plate may become oxidized in use. It is found that the application of a nickel plate following the copper plating as aforesaid does not greatly increase the conductivity or decrease the resistance or breakdown voltage; while at the same time the resistance and breakdown voltage are substantially stabilized at a value approximating that attained by the practice of the process, this value for breakdown voltage being in the range of about 200 to about 500 volts for an igniter as previously particularly referred to.

It is further contemplated that the nickel plating step of the process may be employed whether or not the step or partially reoxidizing some of the copper is used as aforesaid. In either instance, the attained resistance and breakdown voltage prior to the nickel plating step will be little changed by that step of the process and will be stabilized thereby even as against subsequent use of the ceramic element as an igniter in jet engines under conditions of high temperature and in an oxidizing environment.

While there is herein described but one principal process and certain variants and equivalents have been described, it is contemplated that other equivalents and variants will suggest themselves to those skilled in the art from the foregoing particular disclosure. I do not wish to be limited, therefore, except by the scope of the appended claims, which are tobe construed validly as broadlyas the state of the prior art permits.

What is claimed is:- p 1. In the process of making ceramic elements usable in surface gap igniters capable of use at temperatures up to about 2000 F., in which each of said elements has a semi-conductive portion which is formed in part of a "copper oxide and along which a spark is to pass in the 0.03 to 0.04 inch, of at least about 800 volts; the process of treating said elements to reduce said initial breakdown voltage to the range of about 200 to 500 volts, comprising the steps of electrochemically reducing at least about half and substantially less than all of the copper oxide accessible to the surface of the semi-conductive portion of said ceramic elements to form a more conductive portion comprising metallic copper, so as to reduce the breakdown voltage on the basis aforesaid to the range of about 500 to 800 volts; thereafter electrochemically depositing additional metallic copper on the surface portion having the reduced metallic copper thereon by passing a direct current of electricity through an electrolytic cell having an electrolyte containing copper ions, and wherein said semiconductive surface portion of said elements are connected as cathodes, so as to cause some of the copper ions of the electrolyte to deposit onto the semi-conductive portion of said elements, and thereby to reduce their breakdown voltage on the basis aforesaid to the range of about 200 to 500 volts; and stabilizing the electrical resistancein use of said ceramic elements so as to maintain the breakdown voltage thereof substantially unchanged, even when exposed to-relativelyhightemperatures in an oxidizing environment, -'by electrodepositing a metal selected from 'the' group consisting of nickel and rhodium onto the metallic copper on the semi-conductive surface portion of said elements.

'2. The process according to claim 1, in which the reduction of copper oxide and the electrodeposition of copperare carried on as aforesaid to an extent such that the electrical resistance and consequent breakdown voltage across the so-treated surface is lower than that desired as aforesaid, and comprising the additional step of controllably reoxidizing a part of the metallic copper on said surface, so as to increase the electrical resistance across said surface to a predetermined value such that the breakdown v'oltageon the basis aforesaid shall be in the range of about 200 to 500 volts, by heating the ceramic body with the metallic copper to be reoxidized in an oxidizing atmosphere and by controlling the temperature and duration of such heating.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE PROCESS OF MAKING CERAMIELEMENTS USABLE IN SURFACE GAP IGNITERS CAPABLE OF USE AT TEMPERATURES UP TO ABOUT 2000* F., IN WHICH EACH OF SAID ELEMENTS HAS A SEMI-CONDUCTIVE PORTION WHICH IS FORMED IN A PART OF A COPPER OXIDE AND ALONG WHICH A SPARK IS TO PASS IN THE NORMAL USE OF THE DEVICE; AND WHEREIN SOME OF SAID ELEMENTS HAVE AN INITIAL BREAKDOWN VOLTAGE ALONG SAID SEMICONDUCTIVE PORTION, BASED UPON A GAP DISTANCE OF ABOUT 0.03 TO 0.04 INCH, OF AT LEAST ABOUT 800 VOLTS; THE PROCESS OF TREATING SAID ELEMENTS TO REDUCE SAID INITIAL BREAKDOWN VOLTAGE TO THE RANGE OF ABOUT 200 TO 500 VOLTS, COMPRISING THE STEPS OF ELECTROCHEMICALLY REDUCING AT LEAST ABOUT HALF AND SUBSTANTIALLY LESS THAN ALL OF THE COPPER OXIDE ACCESSIBLE TO THE SURFACE OF THE SEMI-CONDUCTIVE PORTION OF SAID CERAMIC ELEMENTS TO FORM A MORE CONDUCTIVE PORTION COMPRISING METALLIC COPPER, SO AS TO REDUCE THE BREAKDOWN VOLTAGE ON THE BASIS AFORESAID TO THE RANGE OF ABOUT 500 TO 800 VOLTS; THEREAFTER ELECTROCHEMICALLY DEPOSITING ADDITIONAL METALLIC COPPER ON THE SURFACE PORTION HAVING THE REDUCED METALLIC COPPER THEREON BY PASSING A DIRECT CURRENT OF ELECTRICITY THROUGH AN ELECTROLYTIC CELL HAVING AN ELECTROLYTE CONTAINING COPPER IONS, AND WHEREIN SAID SEMICONDUCTIVE SURFACE PORTION OF SAID ELEMENTS ARE CONNECTED AS CATHODES, SO AS TO CAUSE SOME OF THE COPPER IONS OF THE ELECTROLYTE TO DEPOSIT ONTO THE SEMI-CONDUCTIVE PORTION OF SAID ELEMENTS, AND THEREBY TO REDUCE THEIR BREAKDOWN VOLTAGE ON THE BASIS AFORESAID TO THE RANGE OF ABOUT 200 TO 500 VOLTS; AND STABILIZING THE ELECTRICAL RESISTANCE IN USE OF SAID CERAMIC ELEMENTS SO AS TO MAINTAIN THE BREAKDOWN VOLTAGE THEREOF SUBSTANTIALLY UNCHANGED, EVEN WHEN EXPOSED TO RELATIVELY HIGH TEMPERATURES IN AN OXIDIZING ENVIRONMENT, BY ELECTRODEPOSITING A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND RHODIUM ONTO THE METALLIC COPPER ON THE SEMI-CONDUCTIVE SURFACE PORTION OF SAID ELEMENTS. 